Traction Control: What is it and how does it work?

By: Ray Johansen
Asst. Instructor -- Advanced Driving & Security Inc.

What is traction, and what does traction control do on my car while I drive? Traction is the grip that your car’s tires have on the road, which is needed to accelerate, turn and brake. If your car, or more specifically, your tires have little or no grip; your car will not accelerate, turn or brake and will skid. Traction control devices in the car will help prevent this loss of grip so your tires will have traction to accelerate, turn and brake.

The most common and well-known traction control device is the Anti-Lock Brake System (ABS). ABS is designed to prevent your wheels from locking up during panic or hard braking. When braking, if your front wheels lock-up, steering control is lost and your car will continue in the same path as before you attempted to brake. The system consists of a host of speed sensors and a central computer. Speed sensors are located at the wheels of the car, which tell the computer how fast they are turning. The computer constantly evaluates the speed of the vehicle and the speed of the wheels. When the brake pedal is depressed and the speed of the wheel reaches or get close to locking-up, the ABS computer will then modulate the amount of brake pressure (or “pump” the brakes), as fast as fifteen times per second, on that wheel. This continuing modulation or pumping will prevent or correct wheel lock-up and allow the driver to brake and steer. ABS systems do vary from auto manufacturer to auto manufacturer. It is important know if the car you are driving is equipped with ABS and how to brake if ABS is activated. Shortly after its introduction in the marketplace, there were insurance industry studies that showed ABS equipped cars were in a higher rate of accidents then those not equipped. In this study, it was shown that the most frequent factor in these accidents was driver error. Drivers were not putting enough pressure on the brake, not holding brake pressure long enough or trying to pump the brakes. Evidence also suggests that drivers were over confident when driving an ABS equipped car and were not allowing for a safe following or braking distance.

To correctly use the brakes in an ABS equipped car in a panic situation, the driver must apply the brakes 100 percent, using all available force. The ABS computer will prevent brake lockup and the tires sliding on the travel surface. This will allow the driver to steer around the threat. It is important to remember that ABS can increase straight-line stopping distances beyond that of threshold braking in a non-ABS equipped car. ABS offers drivers, in an emergency situation, the ability to maintain steering control so they can steer clear of an obstacle or threat. Current ABS systems give feedback to the driver to let them know it is activated and operating during the current braking maneuver. The most common way that ABS communicates to the driver is a pulsing sensation felt in the braking foot or a rattling noise during braking. This is normal operation and is telling the driver ABS is working. As discussed above, do not attempt to modulate the brake yourself and remember to use all the brake force available. The ABS system will take care of the modulation for you and allow you to steer around a threat.

As marketed by most automakers, but related to ABS, is Traction Control. Traction Control is used to prevent the drive wheels from losing grip when accelerating. Spinning tires during hard acceleration may be dramatic, but it is the slowest way to get to your desired speed. Using the car’s ABS speed sensors at the wheels, the Traction Control computer compares the drive wheel’s speed to the car’s road speed. If there is a loss of grip event during acceleration, there are a number of ways that the Traction Control slows the drive wheels so they can regain grip. The most common method is to use the braking system. When the drive wheels lose grip, the ABS computer can apply the brake to the wheel that has lost grip to slow it down so it can regain grip. Another method for slowing the slipping wheels is to reduce the amount of power applied to them. The computer will electronically modify the amount of fuel entering the engine and/or use the transmission to slow the drive wheels so they can regain grip.

The latest traction control technology introduced to motor vehicles is Stability Control. While ABS and Traction Control help manage the grip of the tires for braking and accelerating, Stability Control helps prevent a car from sliding sideways. Using the same components in the ABS and Traction Control systems, Stability Control adds other sensors. These include; steering wheel angle sensor and yaw rate sensor. Yaw is defined as, "the movement of an object turning on its vertical axis”. Stability Control is used to manage the amount of understeer a vehicle will exhibit if the driver used too much steering or entering a turn too fast. It will also manage oversteer if the driver uses too much or too little throttle while turning. Much like the other technologies of traction control, Stability Control will apply the brakes and/or throttle to a wheel or a number of wheels, independently, so the driver can regain control.

How does Stability Control help to regain control of the car? The foundation of ABS and Traction Control were already in place when Bosch pioneered Stability Control with their Electronic Stability Program (ESP) in the Mercedes Benz E Class in 1995.

These same technologies were used to correct an impending slide. Reviewing back, ABS can control individual wheels’ braking forces and Traction Control controls individual wheels’ acceleration forces. Stability Control can use either braking or throttle application to correct a slide. To get a better understanding how braking and acceleration forces affect a cars attitude, we must understand the effect of weight transfer during braking, turning and accelerating. To illustrate, we will use a brake and turn exercise to show how weight transfer will allow the driver to steer more effectively around an obstacle. As we know from previous exercises, to turn more effectively, braking should be done prior to turning. During braking, weight is transferred to the front, “loading up” the front wheels. To take these visuals further; imagine a car with a rod or a pole installed vertically through the roof to the ground of the car. Yaw would be the movement or rotation of the car around this rod. Similar forces are applied to a vehicle when turning. Depending on speed and how much steering is applied, the car can understeer or oversteer.

A realistic scenario would be driving down the interstate at typical highway speeds in the center lane. Imagine what you would do if the truck in front of you accidentally drops its load of gravel on the roadway in front of you. A common evasive maneuver would be to steer around the obstacle, going left then right to avoid the gravel. For this given example, you may put too much steering input when turning left and cause the car to understeer. To correct the understeer, the Stability Control could apply the brake to the inside front wheel. In this case, during the left turn, the inside front wheel would be the left front wheel. This braking of the left front wheel causes the car to get back to the driver’s intended path by reducing the amount of understeer. The deceleration of the vehicle causes weight transfer and puts more weight over the steered wheels. During this evasive maneuver, you are only half done and have not steered back into your lane. When making a quick left/right turn, weight transfer can be quite dramatic and the car can deviate from your intended path. When initially turning left, the weight transferred to the right side of the vehicle. When turning back right and getting back into your lane, the weight will unload from the right side of the car and will quickly transfer to the left. This weight transfer will immediately unload the rear tires and can cause the car to oversteer or fishtail. A common method to prevent oversteer in this scenario, thinking back to the car spinning on the rod, would be to slow the inside rear tire. Slowing down the right rear tire will slow or prevent the rotating of the vehicle on its vertical axis – the rod in our example.

Here are the common marketed names Stability Control is called and their respective manufacturers:

• Audi: Electronic Stability Program (ESP)
• BMW: Dynamic Stability Control (DSC)
• DaimlerChrysler (Mercedes Benz): Electronic Stability Program (ESP)
• Ford Motor Company: Advance Trac
• General Motors: Active Handling System (Corvette)
• Precision Control System (Oldsmobile), Stabilitrak (Pontiac, Buick, Cadillac)
• Jaguar: Dynamic Stability Control (DSM)
• Lexus: Vehicle Skid Control (VSC)
• Porsche: Porsche Stability Management (PSM)
• Volkswagen: Electronic Stability Program (ESP)
• Volvo: Dynamic Stability Traction Control (DTSC)


Modern vehicles, that offer traction control devices, perform a self-test on the ABS, Traction Control and Stability Control computer(s). If there is an error encountered in any of these systems when starting or while driving, an error light will steadily illuminate in the instrument cluster of the vehicle. During this time, one or more of these traction control devices will be unavailable and you should know how to maintain vehicle control if these aids are unavailable. It is also possible that a vehicle make and model may offer switches which power off the ABS, Traction Control or Stability Control. Please familiarize yourself with the vehicle’s controls and owner’s manual. In a later article, we will cover situations where a traction control technology should be switched off.

Remember, these technological advances only aid the driver and cannot defy the laws of physics. As an example, if you are trying to drive 50mph around a corner that your car can negotiate at 30mph, you will go off the road, no matter how many traction control devices your vehicle is equipped with.

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